Optical Switch Assembly and Network Incorporating Same

ABSTRACT

An optical switch assembly comprising at least two optical amplifiers ( 10, 20 ), means for applying a first input signal to one end of both amplifiers ( 10, 20 ) and a second input signal to another end, and means for simultaneously driving one or other of the amplifiers into a saturated state whilst the other is unsaturated such that only the amplifier that is unsaturated provides any significant amplification to the input signals at each end, and means for feeding the amplified output signals from the amplifiers to at least two output nodes such that the two amplifiers ( 10, 20 ) are connected to the two output nodes in opposite connections.

FIELD OF THE INVENTION

This invention relates to optical switch assemblies and to networksincorporating such assemblies. It in particular relates to all opticalswitch assemblies that are suited to switching of packetised datastreams having high bit rates.

BACKGROUND OF THE INVENTION

The very high bandwidth of optical fibres has caused a great increase indata transmission speeds within networks over the past decade. In manysystems the optical signals are converted into electrical signals atconnection nodes. The routing of the data around the network is thenperformed in the electrical domain by analysing the electronic signals.This places a limit on the speeds that can be obtained, since electricalsignal processing and operation of electrical switches is inherentlyslow relative to all optical processing.

Various optical routers are known, perhaps the most widely used beingoptical amplifiers which can be placed at nodes in a network to increasethe transmission length of the network. These devices boost the level ofthe signal, allowing long distances to be traversed despite transmissionlosses. However, such devices do not switch the path of the signalsaround the network.

Several attempts have been made to provide all optical switches but todate the applicant is not aware of any devices which can be successfullyintegrated into an optical transmission network. Devices which useinterferometric principles have been proposed which can perform two bytwo switching, but they are as yet not widely established.

The function of a two by two (2×2) switch assembly is to divert incomingdata streams onto one of two output paths. The switch typically operatesbetween two configurations: a bar state and a cross-over configuration.In the bar configuration the first input stream is passed to a firstoutput path, and the second input stream to a second output path. In thecross-over configuration the first input stream is passed to the secondoutput path and the second input stream to the first path. The state ofthe switch is determined by reading the routing information containedwithin the incoming data packets, typically by electronic processing ofthe signals.

SUMMARY OF THE INVENTION

According to a first aspect the invention provides an optical switchassembly comprising at least two optical amplifiers, means for applyinga first input signal to one end of both amplifiers and a second inputsignal to another end, and means for simultaneously driving one or otherof the amplifiers into a saturated state whilst the other is unsaturatedsuch that only the amplifier that is unsaturated provides anysignificant amplification to the input signals at each end, and meansfor feeding the amplified output signals from the amplifiers to at leasttwo output nodes such that the two amplifiers are connected to the twooutput nodes in opposite connections.

By connected in opposite ways connections we mean that the first end ofone amplifier and the second end of the other are connected together atone node, and the second end of that amplifier and the first end of theother are connected together at the other output node.

The assembly may more specifically comprise at least two semiconductoramplifiers, each having an input node and an output node.

It may further comprise two input data stream nodes adapted to receivefirst and second input data streams.

The drive means may include two control nodes adapted to receive a firstand second pump signals; the inverted pump signal being in anti-phasewith the pump signal; first connecting means for connecting the pumpinput node to the first amplifier input; and second connecting means forconnecting the inverted pump signal to the input node of the secondamplifier.

The drive means may additional include third connecting means forconnecting the first data signal to the first end of both amplifiers;and fourth connecting means for connecting the second data signal to thesecond end of both the amplifiers.

The switch assembly may include first and second output nodes with fifthconnecting means for connecting the first end of the first amplifier tothe second output node, sixth connecting means for connecting the firstend of the second amplifier to the first output node, seventh connectingmeans for connecting the second end of the first amplifier to the firstoutput node; and eighth connecting means for connecting the second endof the second amplifier to the second output node.

Where two optical amplifiers are provided the invention provides an alloptical 2×2 switch capable of operating in either a cross or barconfiguration. The configuration of the switch is determined by thephase of the pump and inverted pump signals applied to the first andsecond amplifiers respectively relative to the phase of the datasignals.

Semiconductor optical amplifiers (SOAs) are essentially laser diodes,without end mirrors, which have fiber attached to both ends. Theyamplify any optical signal that comes from either fiber and transmit anamplified version of the signal out of the other fiber. However, if theamplifier is saturated by a pump signal or inverted pump signal appliedto one end it will not provide any additional amplification to the datastream signal applied to that amplifier. This effectively allows theamplifiers to be controlled such that only one is amplifying at any time(unsaturated) whilst the other is not applying any significantamplification (saturated).

The first amplifier and the amplifier may comprise semiconductor opticalamplifiers having a saturation level lower than the level of the pumpand inverted pump signal applied to the amplifier. The invention maytherefore include a source of pump and inverted pump signals which aresufficient to saturate the chosen amplifiers.

The pump and inverted pump signals may comprise square wave signals.These may be in anti-phase to provide a desired inversion between thesignals. The square wave may have a frequency equal to the rate at whichsignal packets are applied to the inputs of the switch.

The connecting means may comprise one or more optical waveguides, suchas optical fibres.

Where two fibres are required to connect to one input node or to oneoutput node, they may be joined by the use of a polarisation beamcombiner.

The switch may be operable with input signals having a wavelength ofbetween 1500 and 1600 nm, or between a smaller range of 1540 to 1565 nm.The amplifiers may be chosen such that they have an amplifiedspontaneous emission peak within this range, and preferably of around1547 nm. This may be chosen to be close to or the same as the wavelengthof the packetised data that is to be switched. Since the saturation ofthe amplifier does not necessarily depend on the wavelength of the pumpsand the packets, they may be of the same or different wavelengths.

According to a second aspect the invention provides a network comprisingfirst and second input signal lines, each carrying a data signalcomprising packets of data, a switch assembly in accordance with thefirst aspect of the invention having first and second inputs, and agenerator which generates a pump signal and an inverted pump signal, thegenerator generating signals that have the same frequency as the twoinput data streams.

The generator may produce a pump signal in which the duration of thehigh state of the pump signal exceeds the duration of the packets in thedata stream so as to provide a guard time. This may exceed by a shortperiod such as 1 μm.

A single generator may be used to produce both pump and inverted pumpsignals. Alternatively, a separate generator may be used for each one.

BRIEF DESCRIPTION OF THE DRAWINGS

There will now be described by way of example on embodiment of thepresent invention with reference to and as illustrated in theaccompanying drawings of which:

FIG. 1( a) is an overview of a switch assembly in accordance with thefirst aspect of the present invention in a bar configuration, and 1(b)is an overview of the same assembly in a cross configuration;

FIG. 2 is a schematic of a prototype switch assembly used to prove theconcept of the assembly shown in FIGS. 1( a) and (b);

FIG. 3 is a schematic illustration of an optical circuit used to producethe pump signals required by the apparatus of FIG. 2;

FIG. 4 is a schematic illustration of an optical circuit used to producethe packetised data signals supplied to the switch apparatus of FIG. 2;

FIG. 5 is a set of eye diagrams of input packets A and B (top) andoutput packets in both the bar and cross configuration (bottom); and

FIG. 6 is an illustration of BER curves measured at the outputs in boththe bar and cross configurations of the apparatus of FIG. 2.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

An optical switch assembly incorporated into an optical network is shownin FIGS. 1 (a) and 1(b) of the accompanying drawings. The network isadapted to carry packetised data between different nodes in the network,and the assembly is shown at one such node to provide routing of twostreams of packetised data through the node. Each packet within a streamof packetised data generally comprises a data payload to be delivered,and a label carrying routing information. The label is read byprocessing circuitry (not shown) at the node and this information isused to configure the switch assembly. The function of the switch is toroute the two input nodes to the appropriate two output nodes (which issent where being dependent upon the configuration of the switch).

The switch comprises a pair of semiconductor optical amplifiers (10,20)arranged to provide a 2 by 2 assembly having bar and cross connectionconfigurations. The bar configuration is shown in FIG. 1 (a) and thecross in FIG. 1( b). It can be seen that the two input signals Packet Aand Packet B are routed to different output nodes in eachconfiguration-passing straight through in BAR and switching over inCROSS configuration.

In more detail, as shown in FIG. 2, a prototype switching assembly hasbeen constructed and tested. The prototype comprised two semiconductoroptical amplifiers 10,20. These were of the Multi-Quantum Well (MQW)type with a small gain of 31 dB, a saturation power of 13 dBm and an ASE(Amplified Spontaneous Emission) peak at 1547 nm. A first end 11 of thefirst amplifier 10 is fed with a first packet signal 12 (A) and thesecond end 13 with a second packet signal 14 (B), these being the twosignals that are to be routed through the switch assembly. The secondamplifier 20 is connected the same way round, with the first signal 12connected to a first end 21 and the second signal 14 to the second end23. The two counter propagating packet signals flowing through eachamplifier 10,20 are orthogonally polarised to minimise possibleinterference through reflections.

A pump signal (pump 1 or pump 2) is also connected to the first end ofeach of the two amplifiers. The pump signals of the prototype wereproduced using the arrangement of FIG. 3. A two into-one converter isprovided for routing of this signal to the amplifier along with thepacket signal that is already being supplied.

The function of the pump signals is to drive one or other of theamplifiers into a saturated state such that the packet signalspropagating through the amplifier receive little or no amplification. Toensure that only one is saturated at a time one of the Pump signals(pump 2 in this case) is the inverse of the other pump signal (pump1).The pump (or inverted pump) polarisation state input to each SOA ischosen to maximise the effects of cross gain modulation within the SOA.The involved mean power for the signals and pump are −10 dBm and 11.5dBm respectively at the amplifier inputs. The pump low level was alsochosen to be offset from zero so as to fix the working point of theamplifiers to reduce the dynamics of gain recovery. This was found toreduce the signal distortions due to the SOA pattern effects andconsequently to process the high performances at packet rates greaterthan 10 Gbits/s.

In use, at any one time one amplifier will be amplifying the counterpropogating Packet A and Packet B signals propagating through it. Theother will not. To exploit this, the first end of the first amplifier isconnected to one output node along with the second end of the other. Thesecond end of the first amplifier is connected to the other output nodetogether with the first end of the second amplifier. Thus, depending onwhich is amplifying, the output at the nodes may be Packet A on one andPacket B on the other, or may be switched over.

Two polarisation beam combiners (pbc 1 and pbc 2) combine the twoorthogonally polarised Packet signals that are fed to each output node.A band pass filter (bpf1 or bpf2) is located between the combiner andits output node to remove the pump signals from the outputs. Finally anamplifier, such as an erbium doped fibre amplifier, may be provided atthe output node to boost the output to a useful level for onwardstransmission across an optical network.

In the prototype used to prove the concept, the pump signals are createdusing an optical apparatus arranged according to FIG. 3 of theaccompanying drawings, The pump 1 (pump) and pump 2 (inverted pump)signals are square waveforms at λ=1550 nm with a frequency equal to thepacket rate obtained by modulating the output of a continuous wave lasersource 30 using two different Mach Zender (MZ) modulators 31,32 drivenby square wave generators 33,34. In order to allow for the transitiontime of the square waveforms, the high-level part of the square wave ischosen to have a duration that exceeds the packet length. The two MZmodulators are driven in anti-phase to produce the inverted signals. Thesignals from each MZ modulator then pass through a respective Erbiumdoped fibre amplifier 35,36 and a band pass filter 37,38 before passingto the amplifiers.

The test apparatus employed an apparatus for creating the packet signalsA and B as shown in FIG. 4 of the accompanying drawings. The two inputsignals consist of 1.5 μsecond long Non-Return to Zero (NRZ) payloads at10 Gbps, generated by splitting a continuous wave (CW) laser 40 atλ=1548.5 nm modulated through a Mach Zender (MZ) modulator 41 by a(2³¹−1) long pseudo Random bit sequence (PBRS). The packet is producedby a Bit Pattern generator 42 running in burst mode to simulate packetstreams. The output of the MZ modulator is passed through an erbiumdoped fibre amplifier (EDFA) 43 and then a bandpass filter 44 beforepassing to a steerable beam splitter 45,46.

Experimental Results

FIG. 5 of the accompanying drawings shows the input packets A and Btogether with the output packets in both the bar and crossconfigurations of the prototype switch apparatus. The mean power ofreported signals is 6 dBm for input packets and 2 dBm for both packetsat the output ports. The good eye diagram quality of bothconfigurations, and the absence of beating noise at the outputs, confirmthe effectiveness of the scheme.

Finally FIG. 6 of the accompanying drawings shows the BER measurementsat each output port of the switch assembly in both the Bar and Crossconfigurations, in the case of an input wavelength equal to 1548.5 nm.The receiver used to obtain the readings comprised an opticalpre-amplifier with 5 dB noise figure and a photoreceiver whose inputpower was kept to −16 dBm. In this configuration the maximum penalty at10⁻⁹ is lower than 1 dB, which makes the switch assembly suitable forcascaded applications. The performance of the 2×2 all optical switch hasbeen measured also for different signal wavelengths to be lower than 1.5dB in the range λ=1540-1565 nm, making the proposed switch suitable forWM systems.

1-8. (canceled)
 9. An optical switch assembly comprising: at least two optical amplifiers; a first signal path configured to carry a first input signal to a first end of both amplifiers, and a second signal path configured to carry a second input signal to a second end of both amplifiers; a driver configured to simultaneously drive at least one of the two optical amplifiers into a saturated state while the other of the two optical amplifiers is in an unsaturated state, such that only the unsaturated amplifier amplifies the input signals at each end; and a combiner configured to feed the amplified output signals from the amplifiers to at least two output nodes such that the two optical amplifiers are connected to the two output nodes in opposite connections.
 10. The optical switch assembly of claim 9 wherein the two optical amplifiers comprise at least two semiconductor amplifiers, each having an input node and an output node.
 11. The optical switch assembly of claim 9 wherein the driver comprises: two control nodes configured to receive a first pump signal and a second pump signal, respectively, the second pump signal comprising an inverted pump signal that is in anti-phase with the first pump signal; a first connector configured to connect the first pump signal to the input node of the first amplifier input; and a second connector to connect the second pump signal to the input node of the second amplifier.
 12. The optical switch assembly of claim 9 wherein the driver further comprises: a third connector configured to connect the first input signal to the first end of both amplifiers; and a fourth connector configured to connect the second input signal to the second end of both the amplifiers.
 13. The optical switch assembly of claim 9 further comprising: first and second output nodes; a fifth connector configured to connect the first end of the first amplifier to the second output node; a sixth connector configured to connect the first end of the second amplifier to the first output node; a seventh connector to connect the second end of the first amplifier to the first output node; and an eighth connector to connect the second end of the second amplifier to the second output node.
 14. The optical switch assembly of claim 11 wherein the pump signal and inverted pump signal comprise square wave signals.
 15. A network comprising: first and second input signal lines, each carrying a data signal comprising one or more packets of data; a switch assembly configured to receive the packets of data, and comprising: at least two optical amplifiers; a first signal path configured to carry a first data signal to a first end of both amplifiers, and a second signal path configured to carry a second data signal to a second end of both amplifiers; a driver configured to simultaneously drive at least one of the two optical amplifiers into a saturated state while the other of the two optical amplifiers is in an unsaturated state, such that only the unsaturated amplifier amplifies the first and second data signals at each end; and a combiner configured to feed the amplified output signals from the amplifiers to at least two output nodes such that the two optical amplifiers are connected to the two output nodes in opposite connections; and a generator configured to generate a pump signal and an inverted pump signal, the generator generating signals that have the same frequency as the two input data streams.
 16. The network of claim 15 wherein the generator produces the pump signal such that the duration of a high state of the pump signal exceeds the duration of the packets in the data stream so as to provide a guard time. 